1. Field of the Invention
The present invention relates to a method and to apparatus for depositing particles on a moving web of material through the intermediary of a perforated belt which moves over the web.
2. Description of Related Art
The absorbent material used in disposable absorbent products, such as diapers, sanitary napkins, incontinence guards, wound dressings and the like, is normally comprised mainly of absorbent fibres, such as cellulose fibres. Absorbent bodies, or pads, which are comprised mainly of cellulose fibres have certain drawbacks, however. For instance, the ability of such bodies to retain the liquid absorbed is poor, particularly when the absorbent body is subjected to load or pressure. Furthermore, the total capacity of such absorbent bodies to absorb liquid is restricted and such bodies quickly become saturated despite the good liquid-spreading, or liquid-dispersing properties of the fibres.
With the intention of overcoming these drawbacks, there has been developed an absorbent material which has a much greater absorbency than cellulose fibres. These so-called superabsorbents are polymers whose liquid absorption capacity is many times the intrinsic weight of the polymers. Although superabsorbents have good liquid retention abilities, they are unable to spread or disperse liquid to the same good extent. Furthermore, superabsorbents swell while absorbing liquid, thereby forming gels which are liable to deny the liquid access to still unused absorbent material. This may result in so-called gel blockaging.
Superabsorbents may be comprised of different types of polymers and are produced in several different forms, for instance in film or particle forms. This latter form is often used for superabsorbents which are intended as absorbent material for use in disposable diapers, for instance. By "particle form" is meant here all types of material which exist in powder, flake, granular, short-fibre form and like forms.
Different methods for introducing particulate superabsorbents into absorbent bodies have been proposed. "Particulate superabsorbents" will be referred to in the following as "superabsorbent particles" or simply as "particles".
WO 87/00057 teaches a method and an arrangement of apparatus of the kind mentioned in the introduction for forming cavities in a moving fibre web with the aid of a roll provided with a pattern of outwardly-protruding teeth, wherein the cavities thus formed are filled with superabsorbents. The superabsorbents are delivered to the fibre web intermittently, by means of a particle feeder or dispenser mounted above the fibre web, and a scraper which is mounted downstream of the particle feeder and which functions to move those particles which have landed outside the cavities down thereinto. The superabsorbent particles introduced into the cavities are then pressed against the cavity bottom by a second toothed roller having the same tooth pattern as the first roller. The two rollers must therefore be driven exactly synchronously in relation to one another and to the underlying fibre web, in order for the teeth of the latter roller to enter the cavities formed by the teeth of the first roller, and to this end the arrangement includes a drive belt which is provided with a hole-pattern corresponding to the tooth pattern of the rollers and which is intended to pass around said rollers. The drive belt is also arranged for abutment with the fibre web, so as to ensure that no superabsorbent particles will land on the upper side thereof. This known arrangement can therefore not be used to deliver a layer of superabsorbent particles to the upper surface of a material web.
Another method is one of mixing the particles with pulp fibres in the air flow which enters the former. This results in a more or less homogenous mixture of cellulose fibres and superabsorbent particles.
Another method of introducing superabsorbent particles into absorbent bodies involves layering the particles on a material web. This web may be moveable and may be comprised, for instance, of cellulose fluff, tissue, non-woven fabric or the like. The layer of superabsorbent material may then be covered with a further web of material, if desired. Different types of binder can be used to bind the particles firmly to the material web, for instance water or steam. None of the methods known at present for laying superabsorbent particles onto a moving material web will enable sharply defined, discrete particle regions to be obtained when the web is moving at a high speed. This is disadvantageous, of course, since a high production rate is highly significant to production volume.
For instance, it has been proposed to strew superabsorbent particles onto a passing material web, via a rotating roller. An attempt has been made to cover discrete areas or regions of the web with particles, by alternately starting and stopping the roller. An alternative method involves the arrangement of discrete hollows on the peripheral surface of the roller from which superabsorbent particles can be strewn onto the passing web as the roller rotates. None of these methods can be applied satisfactorily in practice at high web speeds when desiring the creation of discrete patterns of application in the form of sharply defined particle regions in absorbent bodies. Neither do these known methods enable their application patterns to be modified or changed in a simple and quick fashion.